Unexpected behavior with dataclasses and weakref

[jbaudisch]

We recently discovered the following error in our project:
TypeError: descriptor '__weakref__' for 'XYZ' objects doesn't apply to a 'XYZ' object

XYZ in our case is a dataclass which uses slots (added v3.10) and weakref_slot (added v3.11) parameters.

We further investigated the error and have come to the following example to reproduce it:

import dataclasses
import weakref


@dataclasses.dataclass(slots=True, weakref_slot=True)
class EntityAsDataclass:
    x: int = 10


class Entity:
    __slots__ = ("x", "__weakref__")

    def __init__(self, x: int = 10) -> None:
        self.x = x


e1 = Entity()
e1_ref = weakref.ref(e1)

e2 = EntityAsDataclass()
e2_ref = weakref.ref(e2)


assert e1.__weakref__ is e1_ref  # works
assert e2.__weakref__ is e2_ref  # fails with "TypeError: descriptor '__weakref__' for 'EntityAsDataclass' objects doesn't apply to a 'EntityAsDataclass' object"

I don't know if this is intended, but we expect EntityAsDataclass and Entity to have the same behavior here.

Thanks!

Linked PRs

• gh-102069: Fix __weakref__ descriptor generation for custom dataclasses #102075
• [3.11] gh-102069: Fix __weakref__ descriptor generation for custom dataclasses (GH-102075) #102662

[sobolevn]

I know how to fix it! Thanks for the report! PR is on its way.